Rails, such as railroad rails, are traditionally fastened down to timber supports, typically timber ties, either directly, or indirectly with the interposition of a tie plate between the rail's foot and the timber support. To hold down the rail on its support, are used either non-threaded fasteners, such as spikes, or, more generally, screw threaded fasteners such as coachscrews. These screw threaded fasteners or coachscrews are screwed into appropriate holes previously drilled into the timber support.
At the end of a certain period of use or after several laying and lifting operations, the screw threaded fasteners tend to shear the fibers of the timber and as a result to compromise the solidity of the fixation of rail on its support.
While this problem of the stability of the screwed-in fasteners in the course of time is less critical when high quality hardwood supports are used, it should be stated that, in the face of a world wide shortage of such hardwood timbers, more and more lower quality woods are even softwood timbers tend to be used.